Stepper motors tend to be particularly prone to vibration and noise because of their inherent stepping motion from one detent position to the next. There have been many designs introduced in the motor industry to reduce motor vibration. For instance, varying the stator pitch angles is the most common way to reduce detent torque, and thus reduce noise and vibration. However, the magnetic coupling between the stator and the rotor creates a nature of vibration that is almost impossible to be eliminated.
For this reason, a mechanical damper has frequently been attached externally to the motor's axial drive shaft in many step motor applications to smooth or isolate any vibration from the load being driven. Such dampers are typically constructed of rubber or other bulk elastomeric material, often in the form of a cylindrical bobbin. It is a very effective way to damp the vibration to obtain smooth motion of the motor's axial drive shaft. The drawback of an external damper is an additional rotating part exposed outside of the motor.
Then, an internal damper was introduced to avoid the rotating part exposed outside of the motor, but this added a requirement for extra space inside the motor, increasing overall motor size. One form of such internal damper couples the rotor to the axial drive shaft through radial spokes or ribs of elastic material for resilient, torsionally flexible action intended to smooth the shaft's rotation as it is driven by the stepping of the rotor. Other internal dampers connect an outer rotor portion to an inner rotor portion with rubber rings or sleeves. The magnetically-active outer portion interacts with the stator to produce stepping motion, while the inner portion is directly coupled to the drive shaft.
Magnetically-coupled dampers, such as those based on Lenz' law and eddy currents are also available, as for example the dampers described in U.S. Pat. No. 9,140,297 to Hashish et al. and U.S. Patent Application Publication 2013/0216351 of Griffin.
US Patent Application Publication 2014/0333159 of Ted Lin (incorporated by reference herein) describes a hybrid stepper motor that locates a permanent ring magnet in the stator assembly radially outside of the stator coils. The magnet flux from this ring magnet remotely magnetizes the rotor, which is positioned radially inside of the stator, mounted to an axial drive shaft.